This application relates to rapid curing of high internal phase emulsions to produce microporous, open-celled polymeric foam materials with physical characteristics that make them suitable for a variety of uses.
The development of microporous foams is the subject of substantial commercial interest. Such foams have found utility in various applications, such as thermal, acoustic, electrical, and mechanical (e.g., for cushioning or packaging) insulators; absorbent materials; filters; membranes; floor mats; toys; carriers for inks, dyes, lubricants, and lotions; and the like. References describing such uses and properties of foams include Oertel, G., xe2x80x9cPolyurethane Handbookxe2x80x9d; Hanser Publishers: Munich, 1985, and Gibson, L. J.; Ashby, M. F., xe2x80x9cCellular Solids. Structure and Propertiesxe2x80x9d; Pergamon Press: Oxford, 1988. The term xe2x80x9cinsulatorxe2x80x9d refers to any material which reduces the transfer of energy from one location to another. The term xe2x80x9cabsorbentxe2x80x9d refers to materials which imbibe and hold or distribute fluids, usually liquids, an example being a sponge. The term xe2x80x9cfilterxe2x80x9d refers to materials which pass a fluid, either gas or liquid, while retaining impurities within the material by size exclusion, interception, electrostatic attraction, adsorption, etc. Other uses for foams are generally obvious to one skilled in the art.
Open-celled foams prepared from High Internal Phase Emulsions (hereinafter referred to as xe2x80x9cHIPEsxe2x80x9d) are particularly useful in a variety of applications including absorbent disposable articles (U.S. Pat. No. 5,331,015 (DesMarais et al.) issued Jul. 19, 1994, U.S. Pat. No. 5,260,345 (DesMarais et al.) issued Nov. 9, 1993, U.S. Pat. No. 5,268,224 (DesMarais et al.) issued Dec. 7, 1993, U.S. Pat. No. 5,632,737 (Stone et al.) issued May 27, 1997, U.S. Pat. No. 5,387,207 (Dyer et al.) issued Feb. 7, 1995, U.S. Pat. No. 5,786,395 (Stone et al.) Jul. 28, 1998, U.S. Pat. No. 5,795,921 (Dyer et al.) issued Aug. 18, 1998), insulation (thermal, acoustic, mechanical) (U.S. Pat. No. 5,770,634 (Dyer et al.) issued Jun. 23, 1998, U.S. Pat. No. 5,753,359 (Dyer et al.) issued May 19, 1998, and U.S. Pat. No. 5,633,291 (Dyer et al.) issued May 27, 1997), filtration (Bhumgara, Z. Filtration and Separation March 1995, 245-251; Walsh et al. J Aerosol Sci. 1996, 27, 5629-5630; published PCT application W/O97/37745, published on Oct. 16, 1997, in the name of Shell Oil Co.), and various other uses. The cited patents and references above are incorporated herein by reference. The HIPE process provides facile control over the density, cell and pore size and distribution, proportion of cell struts to windows, and porosity in these foams.
Economics is an important issue in making HIPE foams commercially attractive. The economics of HIPE foam production depends on the amount and cost of the monomers used per unit volume of the foam, as well as the cost of converting the monomers to a usable polymeric foam (process costs). Making HIPE foams economically attractive can require minimizing one or more of: (1) the total monomer per unit volume of foam, (2) expense of the monomers, (3) the expense of the process for converting these monomers to a usable HIPE foam, or (4) combinations of these factors. The monomer formulation and process conditions must be such that the properties of the HIPE foam meet the requirements for the particular application.
The physical properties of the foam are governed by: (1) the properties of the polymer comprising the foam, (2) the density of the foam, (3) the structure of the foam (i.e. the thickness, shape and aspect ratio of the polymer struts that define the foam cells, cell size, pore size, pore size distribution, etc.), and (4) the surface properties of the foam (e.g., whether the surface of the foam is hydrophilic or hydrophobic). Once the requirements for a particular application are known and achieved, an economically attractive process for preparing the material is desired. A key aspect of this process is the rate of polymerization and crosslinking, together referred to as curing, of the oil phase of a HIPE to form a crosslinked polymer network. Previously, this curing step required that the emulsion be held at an elevated temperature (40xc2x0 C.-82xc2x0 C.) for a relatively long period of time (typically from 2 hours to 18 hours or longer) or the use of pressurized curing (to enable temperatures in excess of 100xc2x0 C.). Such long cure times and/or pressurized reactors can necessitate relatively low throughput rates and resulting higher capital and production costs.
Previous efforts to devise commercially successful schemes for producing HIPE foams have involved, for example, pouring the HIPE into a large holding vessel which is then placed in a heated area for curing. See for example U.S. Pat. No. 5,250,576 (DesMarais et al.) issued Oct. 5, 1993. U.S. Pat. No. 5,189,070 (Brownscombe et al), issued Feb. 23, 1993; 5,290,820 (Brownscombe et al.) issued Mar. 1, 1994; and 5,252,619 (Brownscombe, et al.) issued Oct. 12, 1993 disclose curing the HIPE in multiple stages. The first stage is conducted at a temperature of less than about 65xc2x0 C. until the foam reaches a partial state of cure. Then the temperature is increased to between 70xc2x0 C. and 175xc2x0 C. to effect rapid final curing. The whole process takes about 3 hours. Another scheme to produce HIPE foams envisaged placing the emulsion on a layer of impermeable film which would then be coiled and placed in a curing chamber (U.S. Pat. No. 5,670,101 (Nathoo, et al.) issued Sep. 23, 1997). The coiled film/emulsion sandwich could then be cured using the sequential temperature sequence disclosed in the Brownscombe, et al patents discussed above. U.S. Pat. No. 5,849,805 issued in the name of Dyer on Dec. 15, 1998 discloses forming the HIPE at a temperature of 82xc2x0 C. (pour temperature in Example 2) and curing the HIPE at 82xc2x0 C. for 2 hours. However, none of these approaches offer the combination of very fast conversion (e.g., in minutes or seconds) from HIPE to polymeric foam that would provide for a relatively simple, low capital process for producing HIPE foams both economically and with the desired set of properties. PCT application Serial No. W/O 00/50498, published in the name of DesMarais, et al. on Aug. 31, 2000 describes a process for curing a continuous strip of HIPE into the resulting foam and an inclined tube apparatus for curing HIPE under pressure conferred by the hydrostatic pressure of the emulsion to facilitate rapid curing at elevated temperatures. U.S. Pat. No. 6,274,638 (Yonemura et al.) issued Aug. 14, 2001 discloses a method for producing a HIPE foam in a short period of time by means either of using an active energy ray or by raising the temperature of the HIPE after curing in a continuous process.
The art also discloses using pressure to control the volatility of monomers that, otherwise, would boil off at a suitable polymerization/curing temperature. For example, commonly assigned U.S. Pat. No. 5,767,168, issued to Dyer, et al. on Jun. 16, 1998, discloses the suitability of pressurization to control the volatility of relatively volatile conjugated diene monomers. However, the cure time for the foams disclosed therein is still greater than two hours so there is still substantial opportunity for substantial improvement in curing rate that would improve the economic attractiveness of HIPE foams.
Accordingly, it would be desirable to develop a rapid and efficient process for preparing open-celled polymeric HIPE foam materials with the desired properties without resorting to complex assemblies for containing high pressure needed to cure HIPE at temperatures in excess of the boiling point of water or by adding procedures subsequent to the initial curing process or by adding other complex curing steps such as those comprising e-beam rays, for example.
The present invention relates to a method for obtaining open-celled foams by polymerizing a High Internal Phase Emulsion, or HIPE, which has a relatively small amount of a continuous oil phase and a relatively greater amount of a discontinuous aqueous phase. In particular, the present invention relates to use of more reactive monomers to enable fast curing while also achieving the required physical properties of the foams. The present invention further describes specific initiator systems and levels and curing temperatures which can significantly reduce the time needed to cure the HIPE. This acceleration in curing can significantly reduce capital needs in both batchwise and continuous production of cured HIPE foams while also providing HIPE foams having useful properties comparable to those of foams made with much lengthier or more complex curing processes described in the art.
The process for the preparation of a polymeric foam material of the present invention generally comprises the steps of: A) forming a water-in-oil emulsion from 1) an oil phase comprising specific polymerizable monomers and 2) a water phase comprising an aqueous solution containing from about 0.2 to about 40% of a water-soluble electrolyte; and B) curing the monomer component in the oil phase of the water-in-oil emulsion using a polymerization reaction. The polymerization reaction is conducted at a curing temperature of from about 20xc2x0 C. to about 130xc2x0 C. to form a saturated polymeric foam material. The water-in-oil emulsion will have a volume to weight ratio of water phase to oil phase in the range of from about 8:1 to about 140:1. The oil phase comprises: a) from about 80 to about 99% of a monomer component capable of rapid curing and b) from about 1 to about 20% of an emulsifier component which is soluble in the oil phase and suitable for forming a stable water-in-oil emulsion. Specifically, the monomer component comprises: i) from about 20% to about 97% by weight of a substantially water-insoluble monomer selected from the group consisting of alkyl acrylates, alkyl methacrylates, and mixtures thereof; ii) from about 2% to about 40% of a substantially water-insoluble polyfunctional crosslinker selected from the group consisting of acrylates, methacrylate polyesters, and mixtures thereof; and iii) from about 0 to about 15% of a third substantially water-insoluble monomer. The aqueous phase may also comprise an effective amount of a polymerization initiator system. If desired, after polymerization, the aqueous fraction of the HIPE foam may be removed and the moist foam dried by a variety of techniques to yield the open-celled, microporous, low density product.
The curing of HIPEs in a relatively short time period allows increased production and improved economics relative to previously described methods. Either batch or continuous processes for producing the HIPE can be used.